Considering drainage when planning any garden space is key to ensuring surface water is managed during and after rainfall. Whilst the gardener is concerned with managing water for the health of the plants, the engineer needs to manage the water from hard landscaping to prevent puddles of standing water and, most importantly, make the space safe and comfortable to use by stopping the chance of slipping.

For the team at Horatio’s Garden, safety and comfort was high on their priority list. Horatio’s Garden is a charity which nurtures the wellbeing of people after spinal injury in beautiful, vibrant sanctuaries within the heart of NHS spinal injury centres. The gardens are vital places for reflection and adjustment for people facing life-changing injuries and have a profoundly positive impact on people’s physical and mental rehabilitation.

Craig said, “AWA were approached by the charity in 2020 to create a pavilion that was accessible for patients at The Welsh Spinal Cord Injury and Neuro Rehabiltation Centre in University Hospital Llandough, Cardiff. Part of the brief was to manage the drainage of the hard landscaping within the garden. They needed things like level access to allow people to transition smoothly from indoors to outdoors in a wheelchair. The focus was creating a space where patients could enjoy the garden.

“They had a strict requirement not to have any ponding or puddles on the paths and surfaces. This was for safety, but also for the general comfort and tracking of wheelchair users, who were obviously at the forefront of the garden’s design.

“This made the right drainage solution all the more important. Thankfully the charity was experienced and gave a well-defined brief of their requirements. Our solutions then help to guide the rest of the team as to what they need in order to carry out their work. My first priority was to find out about the soil conditions on the site, which was also going to be relevant for the structural side of the project with the foundations of the pavilion.

“When we start a drainage project we need to make an estimate of how the ground is likely to behave based on soil maps – there’s free information online from the British Geological Society which is really useful. If you click on the map, and it says clay, you’re thinking, “This isn’t going to work”. If it’s chalk, or sand and gravel, you’re looking at a better infiltration potential. After that initial assessment, we need to move on to site testing to prove exactly how the soil behaves; this is called a ground investigation where we ask somebody to undertake a range of soil tests to determine, among other things, how porous it is.

“It would usually be best to pursue an infiltration solution so that the water can drain into soakaways. That’s the easiest, most straightforward way of dealing with surface water, and we need soakage tests to prove if the ground is suitable. What happens is that an external geotechnical professional comes and digs a number of holes on site, they fill them with water and time how long it takes for the water to soak away. It’s peace of mind because the findings are specific to that site and confirmed by a skilled professional from outside of the project. It’s from these results that we’re able to make a judgement as to the next step.

“In this instance, the soakage results were very poor because the soil was heavy clay. This meant the water would not infiltrate as there was nowhere for it to go, so soakaways definitely weren’t an option.

“We were lucky that the site already had a wider drainage strategy in place. The run-off from the buildings was already flowing into a big attenuation tank, so the plan was to tap into that. We had to liaise with the engineers that designed the overall site drainage so we knew we weren’t going to overload this system.

“It was useful to be able to directly engage with the other consultants and their drainage engineers. We approached the idea with them to check if they were happy with it and they said, “Yeah, OK, but give us the numbers. We’ll check them with our system and see what works.” They advised how much extra attenuation was needed, so we were all able to agree on a solution to make it work, which was fantastic.

“Now we knew where the water could go, we could solve another drainage problem – namely making sure the surface of the paths didn’t have puddles or ponding water”

“Normally paths have a slope that allows the water to drain to a gully or channel but this wasn’t possible for Horatio’s Garden, who required flat surfaces for wheelchair users. So, rather than the rain hitting the path and flowing to a low point, it had to hit the surface and soak straight through.

“Thankfully permeable surfaces allow that to happen and generally this solution works well on sites where the ground drains naturally. Unfortunately, that wasn’t suitable for this location because of the clay, so we decided to put an impermeable polythene liner underneath the permeable pavement. This means the water seeps through the path and is then directed to outlets, which are connected to the main drainage. By taking this approach, it allowed us to keep a flat, dry surface that met the client’s requirements without compromising the drainage.

“The Horatio’s Garden project took about a year to complete. I enjoyed coming up with technical solutions for the drainage and paving on what was a challenging site, and it was hugely rewarding to know that patients, their loved ones and NHS staff would be benefitting from the space for years to come.”

Drainage solutions for you

We offer drainage support for large and small-scale projects. Find out how we can help to solve your drainage problem or get in touch directly at mail@awaengineers.com